1. Field of the Invention
The present invention relates to methods of reversing a direction of magnetization in a magnetic film, magnetoresistive films, and magnetic memories using them.
2. Related Background Art
In recent years, semiconductor memories being solid state memories are frequently used in information equipment and there are a variety of types of semiconductor memories including DRAM, FeRAM, flash EEPROM, and so on. These semiconductor memories have both merits and demerits of characteristics and there exists no memory satisfying all the specifications required in the current information equipment. For example, DRAM permits high recording density and many rewriting operations, but is volatile and thus loses information without supply of power. Flash EEPROM is nonvolatile, but takes a long time for erasing and is thus not suitable for fast processing of information.
Under the present circumstances of semiconductor memories as described above, memories making use of the magnetoresistance effect (magnetic random access memories: MRAMs) are promising as memories satisfying the specifications required in many information devices, including the recording time, reading time, recording density, the number of rewriting operations permitted, power consumption, and so on. Particularly, MRAMs making use of the spin-dependent tunneling magnetoresistance effect provide great read signals and are thus advantageous in terms of increase in recording density or in terms of fast reading, and the feasibility of MRAMs is justified in recent research reports.
The fundamental structure of magnetoresistive films used as devices of MRAMs is the sandwich structure in which magnetic layers are adjacent to each other with a nonmagnetic layer in between. Materials often used for the nonmagnetic film are electroconductive materials such as Cu and the like, and insulators such as Al2O3 and the like. Magnetoresistive films using a conductor of Cu or the like as the nonmagnetic layer are called giant magnetoresistive (GMR) films, and magnetoresistive films using an insulator of Al2O3 or the like as the nonmagnetic layer are called spin-dependent tunneling magnetoresistive (TMR) films. Since the TMR films exhibit the magnetoresistance effect greater than the GMR films, the TMR films are preferable as memory devices of MRAMs.
The magnetoresistive films demonstrate a relatively low electric resistance with magnetization directions of the two magnetic layers being parallel, but a relatively large electric resistance with the magnetization directions of the two magnetic layers being antiparallel. Accordingly, it is feasible to read information out by making use of the above nature, using one magnetic layer as a recording layer and the other as a readout layer.
For recording and reproduction of information in MRAMs, it is thus necessary to change the magnetization directions of the magnetic films forming the magnetoresistive film. For changing the magnetization directions, it is necessary, to apply a magnetic field not less than a magnetic field for reversal of magnetization (magnetization reversal field) of the magnetic films.
The magnetization reversal field of the magnetic films will be described below. Main examples of magnetic films with an easy axis of magnetization along the perpendicular direction to the film surface are alloy films and artificial lattice films of rare earth metal and transition metal, artificial lattice films of transition metal and noble metal such as Co/Pt or the like, alloy films with magnetocrystalline anisotropy in the perpendicular direction such as CoCr or the like, and so on. The magnetization reversal field of these magnetic films largely varies depending upon the composition, film-forming methods, and so on. Particularly, the alloy films of rare earth metal and transition metal are greatly affected by the composition. The reason is that the sublattice magnetization of rare earth atoms is antiparallel to that of the transition metal. Namely, when the sublattice magnetizations of the respective metals are substantially equal in magnitude (near the compensation composition), the alloy films have small apparent magnetization and are thus rarely affected by an external magnetic field. When there is a big difference between the sublattice magnetizations of the respective metals on the other hand, the alloy films have large apparent magnetization and are thus largely affected by the external magnetic field. This means that the magnetization reversal field is high near the compensation composition, but the magnetization reversal field becomes lower as deviation becomes larger from the compensation composition. However, if the intensity of apparent magnetization becomes too high, the easy axis of magnetization will deviate from the perpendicular direction because of influence of a demagnetizing field. Among the alloys of rare earth metal and transition metal, GdFe demonstrates small values of the magnetization reversal field, which are approximately several thousand A/m even near the critical composition where the easy axis of magnetization starts inclining from the perpendicular direction. Such values are too large to apply the alloy films, for example, to the memory devices.
In general, when compared with perpendicular magnetic films, longitudinal (in-plane) magnetic films exhibit smaller magnetization reversal fields, but the longitudinal magnetic films tend to increase the magnetization reversal field with decrease in device size. The magnetization reversal field can be higher in the longitudinal magnetic films in certain device size than in the perpendicular magnetic films.
For future increase in density of integration of MRAMs, use of the perpendicular magnetic films as memory devices permits production of magnetoresistive films in small size and is expected to decrease the size of MRAM itself and increase the density thereof, because they are free of problems of curling of magnetization and the aforementioned rise of the magnetization reversal field, which arise in the longitudinal magnetic films.
In the case of the perpendicular magnetic films, however, it is necessary to apply a perpendicular magnetic field strong enough to reverse the magnetization direction in at least one magnetic layer, and thus magnetization reversal methods are still susceptible to study, because the magnetization reversal field of the perpendicular magnetic films is generally high as described above. In the MRAM, it is necessary to record or reproduce information by selecting a specific memory device out of a number of memory cells arrayed in a matrix pattern. There is thus a need for a method of effectively applying the magnetic field to the specific memory device. If the specific device were selected by simply applying a strong magnetic field to the magnetic films, it would be possible to apply the strong magnetic field by supplying a large electric current to a write line or by providing a plurality of write lines per device. However, the flow of the large current in conductor lines is not preferable in terms of heat design or power supply capacity and it is necessary to set the current at least to below a value where the write lines suffer discontinuity due to electromigration. Even within the range below the value, it is desirable to set the current as small as possible, in view of power consumption. The provision of plural write lines per device of magnetoresistive film inevitably increases the device area by that degree, and imposes severe constraints on the layout in the memory cells, thus making high integration difficult.
As described above, in the case wherein the memory devices or the like are constructed of the magnetoresistive film using the perpendicular magnetic films, or even in the case using the longitudinal magnetic films, there will arise issues including the increase in the device area, the increase in power consumption, etc. due to the rise of the magnetization reversal field, in conjunction with future decrease of device size.